Lock-in State Research Articles

Numerical study of an oscillating cylinder in uniform flow and in the wake of an upstream cylinder

Direct numerical simulation is carried out to study the response of an oscillating cylinder in uniform flow and in the wake of an upstream cylinder. It is found that the response of the cylinder wake is either a periodic (lock-in) or a quasi-periodic (non-lock-in) state. In the lock-in state, the vortex shedding frequency equals the forcing frequency. In the non-lock-in state, the shedding frequency shows a smooth variation with the driving frequency. For a cylinder oscillating in uniform flow, a lock-in diagram of different forcing amplitude is computed. However, no clear chaotic behaviour is detected near the lock-in boundary. For a cylinder oscillating in the wake of an upstream cylinder, the response state is strongly influenced by the distance between the two cylinders. By changing cylinder spacing, two different flow regimes are identified. In the ‘vortex formation regime’, found at large spacings, the vortex street develops behind both the upstream and downstream cylinders. The strength of the naturally produced oscillation upstream of the second cylinder becomes important compared to the forced oscillation and dominates the flow, leading to a very small or even indistinguishable zone of synchronization. However, in the ‘vortex suppression regime’, observed at small spacings, the oncoming flow to the downstream cylinder becomes so weak that it hardly affects its vortex wake, and therefore a large zone of synchronization is obtained. The numerical results are in good agreement with available experimental data.

Vibration Response Characteristics of Tandem Circular Cylinders Subjected to Two-Phase Cross Flows. (4th Report. Comprehensive Study on Effects of Bubble Size and P/D).

Based on the experimental data of vibrational response of two tandem circular cylinders, in two-phase cross flows, at the pitch-to-diameter ratios of 1.5 and 3.0 and for average bubble sizes from 3 to 4.0mm and 6 to 7.0mm, comprehensive investigations to the effects of pitch-to-diameter ratio and bubble size on the vibrational response characteristic cylinders were carried out. The following major results are found: (1) for both P/D=1.5 and 3.0, air bubbles caused the cylinder system to excite vibrationally when the flow velocity was smaller than that generating the lock-in state in the two-cylinder system: (2) in higher reduced velocity, air bubbles had suppression and excitation effects on the cylinder system which were dependent on the bubble size and P/D: and (3) in the lift direction of downstream cylinder's vibrathon, the large bubble size and P/D were more effective on the vibration response with respect to the reduction and excitation of vibration, but for thec upstream cylinder's vibration, the small bubble size and P/D were more influential.

B110 円柱の渦励振過程について―離散化渦法によるシミュレーション

The mechanism of the vortex induced vibration of a circular cylinder was discussed based on numerical simulations of the discrete vortex model. The cylinder vibrates even when the frequecy of the vortex shedding is not locked in that of the vibration. Because the vortex sheds into the wake synchronized with the vibration in a regular interval and encourages the vibration in the interval. The process to the lock-in state is triggered by the prevention of the rolling up of the shear layer by the vibration. Therefore, the lock-in phenomenon can be considered as the transition from the vortex shedding by the interaction of the shear layers to the one controlled by the vibration. When the splitter plate is set in the wake, the interaction of the shear layers is prevented and the lock-in state can be occurred in the wider range of flow velocity. As the result, the cylinder can vibrate even in very high velocity.

Vibration characteristics of tandem circular cylinders with a pitch-to-diameter ratio of 1.5 subjected to a two-phase cross flow.

We experimentally examined the vibration response characteristics of tandem circular cylinders with a pitch-to-diameter ratio of 1.5, at which a strong vibrational coupling appears between each cylinder compared with that of 3.0. Experiments were casrried out changing the void fraction from 0% to about 20% (18%∼35%) for each reduced velocity ranging from 2 to 8. The experiments showed the following major results. (1) In tandem cylinders with a pitch-to-diameter (P/D) of, 1.5 air bubbles caused a large vibration in the cylinder system when flow velocity was smaller than that generating the lock-in state in the two cylinders and reduced the vibration slightly for the lock-in velocity range. (2) The response characteristic maps were made to show the effects of air bubbles on the P/D=1.5 tandem cylinder vibration. and (3) Suppression and excitation of the vibration seemed to be caused by the condition of the air bubble distribution in the region between the two cylinders.

MAGNETIC STRUCTURE AND MAGNETOSTRICTION OF EPITAXIAL Er FILMS

MAGNETIC STRUCTURE AND MAGNETOSTRICTION OF EPITAXIAL Er FILMS

Volumetric determination of cuprous oxide: Charles S. Bisson and J. Gordon Sewell, of the University of California ( Jour. Asso. Official Agric. Chem., 1927, 10, 120–124)

In this paper, mode veering, crossing and lock-in phenomena are experimentally analyzed and characterized. Their occurrence is generally found, under different conditions, when there is a parameter variation in the system that produces a change in its behaviour. It often happens that, when the natural frequencies of two modes approach each other, they can cross, veer and eventually present a lock-in state. The problem is analytically investigated for general weakly-coupled two-degrees of freedom systems and experiments, appropriately designed to highlight these phenomena, are presented. In particular, experimental evidence of the damping-dependent transition from veering to crossing is investigated for a two beam system, and experimental lock-in is recalled to show how the gyroscopic systems become unstable when two coupled mechanical parts have the same eigenvalue.